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22. What is a nova?
The word "nova" is part of the Latin
phrase stellar nova, a "new star". At rare intervals through
the years, a bright star has flared up in the skies where no
visible star had ever been before. To the people living upon
the Earth long ago, these were probably new stars. It is only about once every 500 years that one star flares up so brightly.
These are not new stars. They are stars which have been in their places since stars began, but which have met with stellar catastrophes. They have exploded. We do not yet completely understand the process by which a star becomes a nova. Suddenly such a star becomes very bright for a short time and then slowly gets its original lights. It is believed that it explodes deep inside. The cause of such explosions is not known, but they throw tremendous amounts of matter and raise the temperature of the star's surface sometimes by thousands of degrees. Most of the time such explosions do not damage the star, and it returns to its original state.
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23. What happens to the material expelled from a nova?
The material is thrown into space. Sometimes no trace of it can later be seen near the exploded star. Sometimes, as in the case of the Nova Persei, 1901, it remains as a cloud around the star, and forms much dust and gas in space.
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24. What is the origin of planetary nebulae?
At first it was believed that planetary nebulae were shells of gases expelled from a star which had become a nova, as most planetary nebulae have a bright central star, whose radiation makes these gases visible.
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25. What is a planetary nebula?
A planetary nebula is a cloud of gas and dust in space with a more regular form than the gaseous nebulae. The name does not mean that they are connected with planets. Planetary nebulae were so named by Sir William Herschel because, when seen through a telescope, they show a disk. Typical planetary nebulae are the Ring Nebula, in Lyra; the Owl Nebula, in Ursa Major.
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26. Why is the Crab Nebula so called and where is it?
The Crab Nebula is a nebula in the constellation of Taurus. A photograph of the Crab Nebula looks like a crab.
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27. How many novae are there?
Astronomers find about 20 novae every year.
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28. What is a supernova?
A supernova is a nova which is very bright. There have been three stars in the historical past which could be seen even in the day-time. These were the supernova of 1054, whose shell is now known as the Crab Nebula, in Taurus; Tycho's star, in 1572; and Kepler's Nova, in 1604.
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29. Has any star ever become a nova more than once?
Yes. There are several stars which have become nova more than once. T Coronae Borealis exploded in 1866 and again in 1946.
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30. Are there many double stars?
In the list of the 20 brightest stars, seven are double stars.
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31. How many stars are there in a cluster?
There is no limit to the number of stars in a cluster. Some galactic clusters have few stars. Some of the great globular clusters have many hundreds of thousands of stars.
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32. What is a globular cluster?
A globular cluster is a spherical aggregation of stars. There are about 100 globular clusters in our Galaxy. Galactic clusters may have from
10 to about 1,000 stars. Globular clusters have two or three hundred thousand stars.
The globular clusters of our Galaxy are at tremendous distances. The nearest is more than 20,000 light years away.
Some stars that lie in the globular clusters must be tremendously large and bright. The brightest stars in the globular clusters are red stars.
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33. What is meant by a galactic cluster?
A galactic cluster of stars is so called because it takes part in the general rotation motion of the galaxy. They are also known as open clusters. There is no specific form or shape to a galactic cluster. They are, in most cases, within the spiral arms of our own and of other galaxies. The stars in a galactic cluster seem to have a common motion in space. There are about 300 galactic clusters within the part of our own Galaxy visible to us, and there are probably many more clusters that cannot be seen.
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34. What are variable stars?
Variable stars are stars whose brightness is not constant, but whose light changes from time to time.
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